sample essays of IELTS

48 Bacteria

Bacteria are extremely small living things. While we measure our own
sizes in inches or centimeters, bacterial size is measured in microns.
One micron is a thousandth of a millimeter: a pinhead is about a
millimeter across. Rod-shaped bacteria are usually from two to four
microns long, while rounded ones are generally one micron in diameter.
Thus if you enlarged a rounded bacterium a thousand times, it would be
just about the size of a pinhead. An adult human magnified by the same
amount would be over a mile(1.6 kilometer) tall.

Even with an ordinary microscope, you must look closely to see bacteria.
Using a magnification of 100 times, one finds that bacteria are barely
visible as tiny rods or dots. One cannot make out anything of their
structure. Using special stains, one can see that some bacteria have
attached to them wavy-looking hairs called flagella. Others have
only one flagellum. The flagella rotate, pushing the bacteria through
the water. Many bacteria lack flagella and cannot move about by their
own power, while others can glide along over surfaces by some little-
understood mechanism.

From the bacteria point of view, the world is a very different place
from what it is to humans. To a bacterium water is as thick as molasses
is to us. Bacteria are so small that they are influenced by the
movements of the chemical molecules around them. Bacteria under the
microscope, even those with no flagella, often bounce about in the
water. This is because they collide with the watery molecules and are
pushed this way and that. Molecules move so rapidly that within a tenth
of a second the molecules around a bacteria have all been replaced by
new ones; even bacteria without flagella are thus constantly exposed to
a changing environment.